Microwave sintering process

ABSTRACT

A microwave susceptor bed useful for sintering ceramics, ceramic composites and metal powders is disclosed. The susceptor bed contains granules of a major amount of a microwave susceptor material, and a minor amount of a refractory parting agent, either dispersed in the susceptor material, or as a coating on the susceptor material. Alumina is the preferred susceptor material. Carbon is the most preferred parting agent. A sintering process using the bed and novel silicon nitride products produced thereby are described.

This is a continuation of application(s) Ser. No. 08/443,176, filed onMay 17, 1995which, in turn, is a continuation of U.S. Ser. No.08/220,840 filed on Mar. 31, 1994,now both abandoned.

FIELD OF THE INVENTION

This invention relates to microwave sintering processes for ceramicmaterials, to microwave susceptor beds for such processor and tosintered ceramic products having novel, superior properties derived fromthe microwave sintering process.

BACKGROUND OF THE INVENTION

Many ceramic (or ceramic composite) materials are used in the productionof industrial cutting tools and components. Powders of these materialsare typically pressed into shaped preforms which are then sintered athigh temperatures (1000° to 2000° C., depending on the material) todensify and strengthen the tool or wear surface. Silicon nitrideceramics are particularly preferred for industrial cutting tools becauseof their high strength, fracture toughness, wear resistance and hightemperature properties.

Ceramic materials are quite difficult to sinter to nearly full density.Hence a common manufacturing process includes "hot pressing"₁ wherein adisc of the ceramic powder material of interest is pressed in a hightemperature furnace using a mechanical press. The hot pressed disc isthen sliced, diced or core drilled to obtain small ceramic work piecesof the desired shape and size. These are expensive processes.

In the conventional sintering processes, the preform of the ceramicpowder is brought up to its sintering temperature in a radiant heatoven. In order to produce crack-free products, the sintering process isconducted with a slow heating rate. Furnace cycle times are in the orderof many hours. The high temperatures and long heating times can lead toundesired decomposition in the ceramic materials being sintered.

Many ceramic materials are not capable of being sintered to the desireddensities (typically greater than 98% or theoretical density). Expensivepost sintering processes such as hot isostatic pressing are needed.

Most ceramic materials are "transparent" to microwave energy, that ismicrowaves can pass through them. As microwaves pass through theceramic, some energy is absorbed by the ceramic body. This energy isconverted to heat and is capable of heating the ceramic bodyvolumetrically (uniform heating through the volume). Microwave heatingof ceramics has many advantages which derive from a much more rapidheating rate. Higher heating rates can result In better densification.Rapid microwave heating can also reduce the ultimate temperaturenecessary to achieve densification. Improved rapid heating to lowerultimate temperatures can lead to the production of denser ceramicmaterials with finer grain size. These are important features inproducing high strength, wear resistant ceramics.

In spite of the advantages to be gained by microwave sintering, thereare several problems which have hindered its application with ceramicpowders. Many ceramic materials do not couple well with microwaveradiation at low temperatures, that is they are poor microwavesusceptors below about 500° C. Thus, to apply microwave energy forsintering, many ceramics need to be preheated by conduction, convectionor radiation from another source such as a flame or a heating element,or a microwave susceptor material which couples with the microwaveradiation, at least until a high enough temperature is reached, afterwhich the ceramic couples with the microwave radiation. When microwavesusceptors are used as a packed bed around the ceramic or metalmaterials to be sintered, uneven heating is often experienced. Somemicrowave susceptors, such as carbon, become conductors at highertemperatures, which can lead to uneven heating or arcing. Also, as theceramic is sintered, it shrinks due to derisification, and can losecontact with the susceptor bed. Volume shrinkage during sintering isusually about 50 percent. Many microwave susceptors may themselvessinter or fuse together in the susceptor bed, leading to uneven orinefficient sintering of the product. Still other microwave susceptormaterials may decompose, contaminate or react with the material to besintered.

Canadian Patent Application 2,000,109 of Apte et al., laid open on Apr.3, 1991, describes a microwave sintering process for certainnon-susceptor materials such as alpha alumina in a powder bed ofsusceptor materials such as sub-alpha alumina Canadian PatentApplication 2,001,062 of Apte et al., laid open on Apr. 19, 1991,discloses a microwave sintering process for sintering certain ceramicsincluding silicon carbide, silicon nitride and aluminum nitride. Apacked powder bed consisting of a microwave susceptor (ex. metalcarbides, carbon, porcelain, soda-lime glass and barium titanate), anoxygen getter (ex. boron metal carbides, carbon and oxidization), athermal conductor (ex. nitride, aluminum nitride and metals), and aprotective material to generate a localized protective atmosphere (ex.metal carbides, carbon, MoS₂, lead based ceramics). These and otherprior art approaches to microwave sintering of ceramics in powder bedsstill present problems:

1. Many of the prior art processes utilize complex microwave susceptorbeds wherein the materials are chosen to, in situ, form and maintain acontrolled, protective atmosphere during sintering. Thus, for sinteringof silicon nitride, the packed bed might contain silicon nitride.However, using a solid nitride to provide a protective nitrogenatmosphere is problematic since the silicon nitrido powder in the beddecomposes to release nitrogen at the same temperature as the siliconnitride ceramic piece also starts to decompose. The oxygen available atlower temperatures will thus oxidize the ceramic pieces.

2. Many of the powder susceptor beds themselves sinter during thesintering process, creating large gaps in the bed, and uneven orinefficient heating.

3. The use of a packed powder bed to prevent oxygen entering the bedduring sintering necessitates a careful, time consuming packing step.Oxygen trapped in the bed is available for oxidizing the work pieces.

4. The use of other materials such as silicon carbide or carbon, as themain ingredients of a microwave susceptor bed is problematic. Thesematerials become good electrical conductors, and thus poor microwavesusceptors, as the temperature increases during sintering. They can alsoshield the ceramic pieces from the microwave field, i.e. preventmicrowave energy from reaching the ceramic pieces.

5. Many of the materials suggested for use as microwave susceptor bedingredients are expensive ceramics (ex. silicon nitride and boronnitride.).

One prior art approach to microwave sintering of ceramics is to usehigher frequency microwaves (see for example U.S. Pat. No. 4,963,709,issued Oct. 16, 1990, to Kimrey et al.). At these higher frequencies(ex. 14, 28 and 60 GHz), the ceramic material couples with microwaves,for direct sintering. However, the cost of high frequency, specializedmicrowave equipment is prohibitive for most ceramic sinteringapplications. At the commonly used frequencies (915 MHz and 2.45 GHz)equipment is relatively inexpensive and readily available.

There remains a need for an effective microwave sintering process tosinter ceramic and ceramic composite materials.

SUMMARY OF THE INVENTION

The inventors prior experience with powder microwave susceptor bedshighlighted certain of the above problems. Basically, the nature ofpowder susceptor beds gave rise to the need for different beds fordifferent sintering materials. Oxide-type beds, such as hydrated or subalpha alumina beds, were used to sinter ceramic oxides, and non-oxidebeds, such as silicon carbide, silicon nitride and boron carbide, wereused to sinter non-oxide ceramics such as carbides and nitrides. Thepacked powder beds prevented the flow of gases through the bed, thuspreventing the use of protective gaseous atmospheres during sintering.The protective atmosphere had to be provided by including a materialwhich would form a localized protective atmosphere within the bed duringsintering. However, the powder beds occluded a large volume of air(oxygen) which could not escape and thus would react with both thematerial to be sintered and the bed itself.

The inventors discovered a microwave susceptor bed useful for sinteringboth oxide and non-oxide ceramics, and which overcame many of the aboveproblems associated with prior art powder beds. The susceptor bed of thepresent invention is granular such that it forms a porous bed which ispermeable to flowing gases. The bed is formed from a microwave susceptorand a parting agent. The parting agent is functional to prevent fusing,agglomeration or sintering of the susceptor material at high sinteringtemperatures. This creates a free flowing susceptor bed which cancollapse with shrinkage of the material to be sintered, leading to moreuniform and efficient heating. The granular nature of the susceptor bedallows for the direct introduction of a protective gaseous atmosphere,such as by flowing nitrogen, into the susceptor bed during the sinteringprocess. The large granule size results in a susceptor bed with largerinterconnected pores to provide permeability to flowing gases. This hasenabled the use of an oxide material such as alumina, zirconia orthoria, when combined with a parting agent, as a microwave susceptor tosinter both oxide or non-oxide ceramics.

The invention broadly extends to a microwave sintering bed comprising:

granules of;

(a) a major amount of a microwave susceptor material; and

(b) a minor amount of a refractory parting agent either dispersed in thesusceptor materiel or provided as a coating on the susceptor material.Preferred susceptor materials are ceramics such as refractory oxideswhich couple with microwaves between about room temperatures to 2000° C.If the susceptor material does not couple at low temperatures, theparting agent may be chosen to couple with microwaves at these lowtemperatures (up to about 500° C.). Alumina, zirconia and thoria areexemplary susceptor materials. Preferred parting agents are carbon,silicon carbide, molybdenum disulphide and zirconia. The most preferredsusceptor bed is formed from alumina and carbon, alumina being includedin an amount of about 90 to 98 percent weight, and carbon being includedin an amount of about 2 to 10 percent by weight. Preferred granulesizes, in order to create sufficient porosity while preventing excessiveheat loss, are 500 microns to 10 mm, more preferably 0.5 to 3 mm.

The invention also broadly extends to a process of sintering ceramics,ceramic composites or metal materials, comprising;

surrounding the material with a granular susceptor bed, flowing aprotective gas around the material, and irradiating the material and bedwith microwave energy, said bed comprising:

(a) a major amount of a microwave susceptor material, and

(b) a minor amount of a refractory parting agent, either dispersed inthe susceptor material, or as a coating on the susceptor material. Foruniform and efficient heating, the process is most preferably practisedwith the material to be sintered embedded in the susceptor bed and byintroducing a protective gas directly into the bed. When the process ispractised in the sintering of silicon nitride, nitrogen Is the preferredprotective gas.

The invention also broadly extends to a novel form of sintered siliconnitride characterized by:

(a) greater than 95 percent theoretical density;

(b) fine grains which are less than about 1 micron in diameter and lessthan about 5 microns in length; and

(c) a colour which is not darker than light grey.

Commercially available sintered silicon nitride products are generallydark grey to black in colour, indicating a higher percentage of silicondecomposition products are included than are present in products formedby the process of the present invention. The grain size of commerciallyavailable sintered silicon nitride products is generally 1-3 microns indiameter and 10-20 microns in length.

Throughout the disclosure and claims the term "microwave susceptor" ismeant to include a material that couples with microwaves to the extentthat it will raise the temperature of the material to be sintered eitherto the desired sintering temperature or at least to a temperature atwhich the material to be sintered couples with microwaves.

Throughout the disclosure and claims, the terms "granules" or "granular"are meant to denote agglomerates or pellets and like of powderedparticles, shaped and sized so that a bed of the granules is freeflowing and relatively permeable to flowing gases. These terms aredistinct from powder materials, which allow very limited gas movement bydiffusion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical cross-sectional view of the assembled, insulatedmicrowave susceptor bed of the present invention, showing the granularmicrowave susceptor bed surrounding the work pieces to be sintered:

FIG. 2 is the same view as FIG. 1, after sintering, showing theshrinkage of the work pieces in the free flowing granular microwavesusceptor bed;

FIG. 3 is a vertical cross-sectional view of the assembled insulatedmicrowave susceptor bed wherein the ceramic work pieces are housedwithin an internal crucible in the granular microwave susceptor bed; and

FIG. 4 Is a plot of flank wear against cast iron material removed in acutting wear performance test, comparing silicon nitride tools sinteredin accordance with the process of the present invention (top line)against three commercially available silicon nitride tools (bottom threelines).

DESCRIPTION OF THE PREFERRED EMBODIMENT

The microwave susceptor bed of the present invention includes at leasttwo components:

(a) a major amount of a microwave susceptor material; and

(b) a minor amount of a refractory parting agent dispersed in, orcoating, the microwave susceptor material.

The microwave susceptor material is chosen according to the material tobe sintered. It should be stable and microwave susceptible at the hightemperatures of sintering. Most preferred susceptors are refractoryoxides including alumina, zirconia and thoria. Alumina is mostpreferred. Alpha alumina and alumina hydrate are the preferred forms.Hydrated alumina is a good microwave susceptor from room temperature toover 2000° C. Alpha alumina does not couple well at room temperature,but couples very well above 400° C. When alpha alumina is used as themicrowave susceptor, the parting agent should be chosen to providemicrowave coupling from room temperature up to about 400° C. If zirconiais used, cubic or tetragonal zirconia are preferred forms.

Zirconia and thoria are less preferred because of their higher cost,without providing a heating advantage over alumina.

The parting agent is a refractory material which, when included in aminor amount, prevents the susceptor material from substantialsintering, agglomerating or fusing at high temperature, thus creating afree flowing susceptor bed, even at the high sintering temperatures. Theparting agent is a refractory which is stable at the high sinteringtemperatures, that is it does not decompose or react with either thesusceptor material or the material to be sintered. Carbon, siliconcarbide and zirconia are preferred parting agents, carbon being the mostpreferred. Impurities in the susceptor bed materials such as oxides ornitrides (ex. Fe₂ O₃, SIO₂, BN) that melt or react below the sinteringtemperatures to be reached are detrimental and should be kept belowabout 1 or 2 percent by weight.

The susceptor bed is granular, formed of agglomerates or pellets havinga size such that a high percentage porosity exists in the bed(preferably greater than 30 percent, most preferably about 50 percent).The granules are formed from fine powders by known pelletizing oragglomerating processes, typically in disc or drum pelletizers. Thesusceptor and parting agent materials are tumbled with appropriatebinders, such as polyvinyl alcohol, and water until the desired particlesize is obtained. If the parting agent is to be generally dispersedthroughout the particles, the susceptor material is tumbled with theparting agent. If the parting agent is a coating on the susceptormaterial, it is added after the susceptor is agglomerated to the desiredgranule size. Multiple coatings may be used. The granules may also beformed by making a paste out of powders of the susceptor and partingagent materials, extruding the paste into various shapes and cutting upthe extrudates to the desired size.

Generally, the granules have a spheroidal or cylindrical shape, althoughother irregular shapes may be used, provided they form a free flowingmaterial. A granule size range of about 500 microns to 10 mm ispreferred. More preferably, a granule size of 0.5 to 3 mm is used. Mostpreferably the granule size is about 1 to 3 mm.

The amount of parting agent used is a minor portion of that of thesusceptor material, so as to impart the free flowing property to thegranules without detracting from the heating ability of the susceptormaterial. Generally, less than 10 percent by weight is needed. Whencarbon is the preferred parting agent, and alumina is the preferredsusceptor material, the amount of parting agent is preferably in therange of 2-10 percent by weight. Significantly higher amounts may leadto uneven heating as carbon conducts at high sintering temperatures.However, at lower temperatures carbon itself couples with microwaves,providing some assistance in heating.

The free flowing granular microwave susceptor bed of the presentinvention is useful in sintering a wide range of products, includingceramics, ceramic composites and metal powders. The bed is particularlyuseful in sintering ceramic nitrides, including silicon nitride andaluminum nitride, ceramic composites such as aluminum oxide and titaniumcarbide, and metal powders such as nickel and copper powders. Thematerial to be sintered may exist in a variety of forms/shapes. Forexample, cutting tools of ceramics such as silicon nitride are formed aspressed powder preforms.

The material or work pieces to be sintered may be embedded directly inthe granular susceptor bed (which is most preferred) or may be sinteredon top of the granular bed or in a microwave transparentcrucible/container within the susceptor bed. Many ceramic materials,such as silicon nitride, must be protected from the environment duringsintering to prevent the formation of undesired decomposition orreaction products such as oxides. In such cases, a protective gaseousatmosphere is preferably provided around the material to be sintered.When the material to be sintered is either embedded in the granularsusceptor bed, or placed on top of the bed, the protective gaseousatmosphere is provided by directly introducing a protective gas into thebed below or adjacent the material to be sintered. When the material tobe sintered is housed in a separate crucible/container, the protectivegas is introduced into the crucible/container. Any nonreacting gascapable of protecting the material to be sintered may be used. Typicallynitrogen, hydrogen or argon are used.

The microwave equipment used to achieve the microwave sintering processis generally conventional. The microwave equipment consists of amagnetron and a resonant cavity connected by a waveguide. Within theresonant cavity is housing, which holds the microwave susceptor bed andthe work pieces to be sintered. Normally, microwave radiation in aconventional microwave oven is in a frequency of 2.45 GH. Lowerfrequencies might be utilized. At higher frequencies, microwave couplingwith the sintering material is not problematic, so the invention haslittle application.

FIG. 1 shows the housing 10, communicating with a microwave waveguide12. The housing 10 consists of a metallic applicator (container) 14, anda removable metal cover 16, so arranged to prevent microwave leakage. Aquartz window or hole (not shown) exists between the waveguide and theapplicator 14 to allow for passage of the microwaves into the applicator14. The applicator 14 is lined with microwave transparent insulation 18such as ceramic fibre "CER-WOOL" HTZ8 (Premier Refractories andChemicals Inc., King of Prussia, Pa, U.S.A.). The work pieces 20 to besintered and the granular microwave susceptor bed 22 are loaded intocavity within the insulation 18. No packing of the susceptor bed isnecessary. The granular susceptor bed 22 is simply poured around thework pieces 20 as they are added in multiple layers. Further microwavetransparent insulation is then laid on top of the susceptor bed 22. Oneor more gas inlet tubes 24 extend through the metal cover 16 to the baseof the susceptor bed 22. Nitrogen may escape from the susceptor bedthrough the fitting cover 16 or through holes (not shown) drilled in themetal applicator to enable gas flow.

FIG. 2 illustrates the microwave susceptor bed of the present inventionafter sintering. When compared to FIG. 1, it will be noted that thesintered work pieces 20 have shrunk during sintering, and the freeflowing granular bed 22 has collapsed around the work pieces 20.

FIG. 3 shows a less preferred arrangement of the susceptor bed 22 in thehousing 10. This set up is most suitable when the materials to besintered would be affected by even trace contamination coming from thesusceptor bed, or when the materials to be sintered need to be exposedto carefully controlled atmospheres which could react with the susceptorbed. A crucible 26 is embedded within the susceptor bed 22 in order tohouse the work pieces 20. The crucible 26 may be made from any materialwhich is translucent to microwave energy, is refractory, stable atsintering temperatures and does not interact with the work pieces to besintered. Alumina and quartz are preferred. The crucible has a fittingcrucible cover 28. The work pieces 20 may be placed in a single layer,or stacked within the crucible 26. Microwave transparent insulation 18is placed above the work pieces within the crucible 26. The gas inlettube(s) 24 extends into the crucible 26 to ensure a gas flow around thework pieces 20. The susceptor bed 22 is covered with microwavetransparent insulation 18.

The microwave sintering bed of the present invention has importantadvantages over the prior art:

1) The free flowing granular bed eliminates the need for careful andtime consuming packing of either the ceramic pieces or a powdersusceptor bed around the pieces to be sintered.

2) The free flowing bed follows tie shrinking work pieces duringsintering, allowing the pieces to be heated uniformly until they arefully sintered.

3) The porous, permeable nature of the bed allows the work pieces to beprotected against decomposition by introducing a protective gaseousatmosphere into the bed during sintering.

4) The preferred microwave susceptors, alumina, zirconia and thoria, arecapable of rapid, uniform heating to temperatures above 2000° C.

5) The microwave susceptor bed is capable of sintering a large range ofceramic and ceramic composite pieces. The susceptor bed does not have tobe altered for each type of material to be sintered, as with many of theprior art approaches.

6) The microwave susceptor bed enables the work pieces to be sintered toreach a high density, at least as great as 95%, and typically greaterthan 98%. This eliminates the need for expensive post sinteringtreatments such as HIPping.

7) Rapid heating with microwave process of the present invention canlead to improved properties in the sintered material. In particular, theinvention is able to produce silicon nitride with higher density, finergrain size and lower percentage of decomposition products, than achievedby prior art processes.

In addition to ceramics and ceramic composites, the susceptor bed of thepresent invention has been demonstrated in sintering bodies made frommetal powders. Metal powders often do not couple well with microwaveenergy, but can be heated indirectly by microwaves in the susceptor bedof the present invention.

The invention is further illustrated by the following non-limitingexamples.

EXAMPLES 1

This example describes one process for making the granular susceptor bedfor microwave heating and its ability to heat to high sinteringtemperatures.

Five kilograms, hydrated alumina, -200 mesh in size was mixed with 500 gcarbon black having a mean particle size of about 1 micron. The two wereagglomerated in a disc pelletizer called an Eirich Mixer (EirichMachines Ltd., Maple, Ontario, Canada). The agglomeration process wasassisted by using polyvinyl alcohol (50 mL for 5 kg alumina) as a binderto provide strength to the agglomerates. Suitable quantities of water(about 500 ml) were sprayed onto the powder mix in a well establishedprocedure provided by the manufacturer of the equipment. Theagglomeration was stopped after 30 minutes when the agglomeratesappeared to be about 2 to 3 mm in size (on average) hut none weremeasured to be in excess of 10 mm in size.

The agglomerates were dried out In a pan at 70° C. for 24 hours. The dryagglomerates were placed In a microwave applicator lined with CERWOOLinsulation in a microwave field. The agglomerates were subjected tomicrowave energy starting at 500 W and the energy was increased every 10minutes by 100 W to 1400 W. The temperature at the centre was measuredas 1850° C. After microwave heating, the granular susceptor bed remainedas distinct, free flowing granules, that is the agglomerates did notadhere to each other. Similar results were obtained when alpha aluminapowder was substituted for hydrated alumina. This example shows that thesusceptor bed is capable of generating temperatures or 1850° C.,adequate to sinter many ceramic, metal and composite materials. The freeflowing nature of the susceptor bed before, during and after thesintering process demonstrated the ability of the bed to collapse aroundmaterials to be sintered.

EXAMPLE 2

This example shows an alternative method for making a suitable granularmicrowave susceptor bed in accordance with the present invention.

Five kilograms of hydrated alumina was agglomerated as in Example 1using 50 mL polyvinyl alcohol as a binder and 500 mL of water. Once thealumina had agglomerated to about 2 or 3 mm spherules, carbon powdersimilar to the one used in Example 1, was added slowly while theagglomerator drum continued to rotate. A total of 250 g of carbon wasadded. The alumina spherules were gradually coated with carbon. Afteranother 15 minutes, the process was stopped and the spherules were driedat 70° C. for 24 hours.

These agglomerates were placed in a microwave field as in Example 1 andwere subjected to a maximum of 1400 W. Once again, a maximum temperatureof 1850° C. was obtained. It was observed that the spherules were looseat the end of the experiment. Similar results were obtained when alphaalumina was substited for alumina hydrate.

EXAMPLE 3

This example shows that the role of the carbon as a parting agent forthe spherules to prevent the formation of lumps.

Spherules of hydrated alumina (without the carbon parting agent) wereprepared as in Examples 1 and 2 and dried in an oven at 70° C. for 24hours. These agglomerates were placed in a microwave applicator andsubjected to microwave power from 500 to 1500 W as in the previousexamples. After 1200 W power was reached, it was noticed that there werehot spots within the susceptor bed, as evidenced by uneven lightemission seen through the Fibrefrax Insulation. At higher powers therewere instabilities in microwave operation leading to an increase inreflected power. The temperature measured in the bed varied from 1400°C. to over 2000° C. (as evidenced by damage to the sapphire sheath usedin the temperature sensor). After the assemblage was cooled, someagglomerates remained separated while others had lumped and even fusedtogether. These lumps are deleterious since their larger size limits theability of the susceptor bed particles to flow into the void spacescreated by the shrinkage of the materials to be sintered in the bedduring the microwave process. This limits heat transfer and heatingefficiencies of the sintering process. Also, hot spots in the bed causeundesired decomposition products to form in the sintered materials.

EXAMPLE 4

This example shows the use of the susceptor bed for sintering siliconnitride.

An assembly of insulators, silicon nitride samples, and susceptor bedwas prepared as follows:

The granular susceptor bed was made using the procedure indicated inExample 1. One kilogram silicon nitride powder (Ube, SNE-10, 0.3micrometer mean diameter) was mixed with 5 wt % alumina (Alcoa A-16 SG.0.5 micrometer mean diameter) and 5 wt % yttria (H.C. Starck grade Cfine). The mixing was performed using hexane as a liquid medium andalumina balls (1/4" diameter) as dispersing media, on a ball mill for 16hours. The mixture was dried in air at room temperature, and the driedpowder was compacted in a tool steel die at a pressure of 40,000 psi(275 MPa) using a manually operated hydraulic press (Carver laboratorypress, Model M). The compacted powder was further treated by leoetaticpressing at 60,000 psi.(410 MPa) The "green" compact so prepared has abulk density of 1.72 g/mL which is 53% of the theoretical value for thiscomposition.

The insulation, susceptor bed and the ceramic pieces were assembled asshown in FIG. 1. The system was purged with nitrogen for 10 minutes andthe flow was then reduced to about 0.5 mL per minute. The assemblage wasenergized with microwaves starting at 500 W and then increasing thepower by 100 W every 10 minutes. When the power reached 1200 W it washeld constant for 20 minutes and then shut off. The assemblage wasallowed to cool under flowing nitrogen. It was found that the siliconnitride compacts had shrunk and a density of 3.2 g/mL was measured. Thisdensity is 98.5% of the theoretical value for this composition. Thesintered pieces appeared light grey in colour and had acicular grainsabout 0.3 microns diameter and 1 to 2 microns long.

Silicon nitride, when sintered using the sintering aids of the type usedin this example, forms acicular (needle shaped) grains, with a crystalstructure known as beta silicon nitride. The microstructure of such agrain structure is specified by the average diameter and length of theindividual needles. Sintered silicon nitride crystals in commerciallyavailable cutting tools were measured to have grain sizes of about 1-3microns in diameter and 10-20 microns in length.

The colour of the sintered pieces from this example (after polishing)was light grey, compared to the dark grey or black appearance ofcommercial silicon nitride cutting tools. On a standard colour chart,for instance the Rock-Color Chart of the Geological Society of America(printed by Huyskes-Enschede, Netherlands), the work pieces formed bythe present invention were no darker than light grey, between N7 and N8in the chart. Commercial silicon nitride cutting tools were dark grey toblack, between N2 and N3 on the colour chart.

When the test was repeated with a susceptor bed made following thepractice of Example 2, the results obtained were similar.

EXAMPLE 5

This example shows the use of the susceptor bed for sintering aluminumnitride.

An assembly of insulators, aluminum nitride compacts was prepared Asfollows:

The susceptor bed was made following the procedure outlined inExample 1. Two hundred grams of aluminum nitride obtained from TokuyamaSoda. (Grade F) was mixed with 3 wt % yttrla (H.C. Stark Grade C fine).The mixing was performed using hexane as a liquid medium and aluminaballs (1/4" diameter) as dispersing media on a ball mill for 16 hours.The mixture was dried in air at room temperature (25° C.). Powder fromthe dried batch was compacted in a tool steel die at a pressure of40,000 psi (275 MPa) using a manually operated hydraulic press (Carver,Laboratory press model M ). The powder compact was further treated bycold isostatic pressing at 60,000 psi (410 MPa). The green compact had adensity of 1.67 g/mL which is 51% of the theoretical value for thiscomposition.

The insulation, the susceptor bed and the ceramic pieces were assembledas shown in FIG. 1. The system was purged with flowing nitrogen forabout 10 minutes and then the flow was reduced to about 0.5 mL/minute.The assemblage was energized with microwaves starting at a power inputof 500 W and then increasing the power by 100 W every 10 minutes. Whenthe power reached 1200 W it was held constant for 20 minutes and thenshut off. The assemblage was cooled in flowing nitrogen. It was foundthat the aluminum nitride compacts had shrunk to about half theiroriginal size and had a sintered density of 3.18 g/mL. The density is97% of the theoretical value for this composition.

When the procedure was repeated using the susceptor bed described inExample 2 the results obtained were similar to these. Normal sinteringof aluminum nitride is generally performed over 24 hours in electricalfurnaces requiring 25 to 90 kW power supplies.

EXAMPLE 6.

This example illustrates the beneficial effect of microwave sinteringwith respect to any side reactions occurring in the material to besintered during convention sintering.

Silicon nitride ceramics are sintered at temperature in the range of1750° to 1850° C. In this temperature range, silicon nitride candecompose releasing silicon and nitrogen gas. The following featureshave been documented:

the extent of decomposition increases with increasing temperature; and

the extent of decomposition increases with the duration which theceramic material experiences the high temperature.

the release of silicon in minute quantities (<0.001%) causes siliconnitride to turn black in colour.

Microwave sintered silicon nitride ceramics produced following thepractice in Example 4 appear light grey in colour. This compares to thedark grey to black colour of commercially available silicon nitrideproduced by conventional sintering and hot pressing.

Green ceramic compacts of silicon nitride formed as in Example 4 wereheated in an electrical resistance furnace to 1800° C. over a period of12 hours and soaked for a period of 4 hours in a nitrogen atmosphere. Oncooling it was found that the silicon nitride pieces were dark black incolour.

This shows that microwave processing in accordance with the presentinvention significantly reduces or eliminates the decomposition ofsilicon nitride during sintering.

EXAMPLE 7

This example illustrates the beneficial effect of rapid heating in termsof the grain size of the sintered product.

Silicon nitride compacts were prepared and sintered following thepractice described in Example 4. After the samples were sintered, thesintered pieces were cut. polished, an etched in a microwave plasma inan atmosphere of fluorocarbon gases. The samples were examined under ascanning electron microscope. The micro structure revealed very finegrains.

The grains of sintered silicon nitride appear as elongated needles. Thesample examined revealed an acicular structure with needles 0.3 to 0.5microns diameter and 1 to 2 microns long,

Samples of commercially available silicon nitride ceramics (obtainedfrom Kennametal Inc. of Latrobe, Pa., U.S.A, and designated as KY-2000)were similarly prepared and generally revealed a structure with needlesbetween 1 and 3 microns diameter and 10 to 20 microns long.

This reveals the fine grained feature of microwave sintered siliconnitride ceramics prepared in accordance with the present invention.Finer grained material wears at slower rates than coarse grainedmaterial under similar test conditions. Finer grained materials alsohave more uniform properties such as hardness and toughness.

EXAMPLE 8

This example shows the beneficial effect of microwave sintering inaccordance with the present invention on the properties of siliconnitride as demonstrated by the wear performance during single pointturning on a high speed lathe.

Silicon nitride samples were microwave sintered using the practicedescribed in Example 4. The sintered pieces were ground to producecutting tools which meet the standard tool specification RNGN-45, T6(ASA standards),

These samples were evaluated at the National Research Council in Ottawain single point turning on a high speed lathe. The material machined wasgray cast iron. The machining parameters were as follows:

    ______________________________________                                        speed        2,000 surface feet/minute (600 m/min)                            feed         0.015"/revolution (0.375 mm/rev)                                 depth of cut 0.050"/pass (1.250 mm/pass)                                      ______________________________________                                    

Similar tests were also performed on three commercially availablecutting tool inserts from USA (KY 2000, Kennamental, Latrobe, Pa.), fromAsia (marketed by Newcomer Products Inc., designated as "Newpro Exp"),and from Europe (Grade 690, Sandvik, Stockholm, Sweden). The results areshown in FIG. 4. The superiority of the microwave sintered inserts isclear when it is realized that tools have to be changed when their wearreaches 0.020 inches (0.5 mm).

EXAMPLE 9

This example shows the limitations of using carbon alone, or and itsrelated materials such as graphite and silicon carbide, for makingsusceptor beds.

A experiment similar to the one shown in Example 4 was repeated but thesusceptor bed was just the carbon black powder. After using the samepower for the same duration as in Example 4, the silicon nitride sampleswere sintered to densities less than 70% of the theoretical value,indicating that the necessary temperatures were not achieved.

In another variation of the same experiment the silicon nitride compactswere placed in a graphite crucible in order to increase the mass of thematerial that generated the heat from the microwave energy. Theassemblage was subjected to the same power cycle as in Example 4. Thesilicon nitride samples had sintered to a density of 2.5 g/mL which isabout 78% of the theoretical value for this composition.

It is clear from these examples that the use of carbon alone as asusceptor bed limits the maximum temperature that can be achieved tovalues much less those required for sintering ceramics.

EXAMPLE 10.

This example shows the use of susceptor bed of the present invention forsintering ceramic composite compacts made of aluminum oxido and titaniumcarbide.

A batch of Al₂ O₃ mixed with TiC powder was prepared by ball millingalumina (Alcoa. A16 S.G.) and titanium carbide (H.C.Stark fine grade) ina nalgene jar with hexane as a dispersing liquid and alumina balls (1/4inch dia.) for 16 hours. The powder mixture was dried and compacted in amanually operated hydraulic press (Carver, Laboratory press, model M) ata compaction pressure of 40,000 psi (275 MPa) and further isopressed at60,000 psi (410 MPa) in a cold isostatic press.

The ceramic compacts were assembled following the practice of Example 4in an insulation and susceptor bed and then subjected to microwave powerof 500 watts. The power was increased to 1500 watts at the usual rate of100 watts every 10 minutes. The system was held at peak power for 25minutes and then cooled slowly to room temperature.

The compacts had sintered to a density of 4.00 g/mL which is about 95%of the theoretical value for this composition.

All publications mentioned in this specification are indicative of thelevel of skill of those skilled in the art to which this inventionpertains. All publications are herein incorporated by reference to thesame extent as if each individual publication was specifically andindividually indicated to be incorporated by reference.

The terms and expressions in this specification are used as terms andexpressions of description and not of limitation. There is no intention,in using such terms and expressions, of excluding equivalents of thefeatures illustrated and described, it being recognized that the scopeof the invention is defined and limited only by the claims which follow.

We claim:
 1. A microwave heating assembly for use in heating orsintering a material, said microwave heating assembly comprising:A. amicrowave heating bed of granules comprisinga. a major amount of amicrowave susceptor material selected from the group consisting ofalumina, zirconia, thoria and mixtures thereof; and b. an amount lessthan about 10% by weight of a refractory parting agent either dispersedwith the susceptor material or as a coating on the susceptor material;and B. the material to be heated or sintered surrounded by the granules,which material is selected from the group consisting of ceramics andmetal materials.
 2. The microwave heating assembly as set forth in claim1, wherein the parting agent is selected from the group consisting ofcarbon, molybdenum disulphide, silicon carbide and zirconia.
 3. Themicrowave heating assembly as set forth in claim 1, wherein thesusceptor material is alumina and wherein the parting agent is carbon.4. The microwave heating assembly as set forth in claim 3, wherein thealumina is included in an amount in the range of about 90-98 percent byweight, and wherein the carbon is included in an amount in the range ofabout 2 to 10 percent by weight.
 5. The microwave heating assembly asset forth in claim 4, wherein the granules of the microwave heating bedhave a size in the range of about 0.5 to 3 mm.
 6. The microwave heatingassembly as set forth in claim 1, wherein the granules of the microwaveheating bed have a size in the range of about 500 microns to 10 mm. 7.The microwave heating assembly as set forth in claim 1, wherein thegranules of the microwave heating bed have a size in the range of about0.5 to 3 mm.